Optical fiber transmission- line

ABSTRACT

A dispersion-managed optical fiber transmission-line with which the formation of side bands around the optical signal wavelength can be suppressed even when carrying out high-speed signal transmission.  
     An optical fiber transmission-line  1  constitutes one repeater span installed between a transmitter (or repeater)  2  and a receiver (or repeater)  3  and is made up of N sections  4   1  through  4   N  in sequence from the transmitter  2  to the receiver  3.  The chromatic dispersion at the wavelength 1.55 μm is positive in the sections  4   n  where the value of n is odd and is negative in the sections  4   n  where the value of n is even. The ratio between the maximum value and the minimum value of the average chromatic dispersions D n  of the sections  4   n  is at least 1.3 and not greater than 10.0.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to an optical fiber transmission-line fortransmitting multiple wavelength optical signals in a wavelengthdivision multiplexing transmission system.

[0003] 2. Related Background Arts

[0004] A wavelength division multiplexing (WDM) transmission system caneffect high speed, high capacity optical communication by transmittingmultiple wavelength optical signals. Because the transmission loss of asilica optical fiber used as an optical fiber transmission-line is smallaround the wavelength 1.55 μm, and optical amplifiers for amplifyingoptical signals in the wavelength 1.55 μm band are available, multiplewavelength optical signals in the wavelength 1.55 μm band are used inWDM transmission systems.

[0005] When in an optical fiber transmission-line for transmittingmultiple wavelength optical signals there is chromatic dispersion in theoptical signal wavelength band, the pulse waveform of the optical signalcollapses and transmission quality deteriorates. Therefore, from thispoint of view, it is desirable for the chromatic dispersion in theoptical signal wavelength band to be small. On the other hand, when thechromatic dispersion in the optical signal wavelength band issubstantially zero, the nonlinear optical phenomenon of four-wave mixingtends to occur, crosstalk and noise arise, and transmission qualitydeteriorates. The occurrence of four-wave mixing can be suppressed bymaking repeater spans short and reducing optical signal power, butbecause this makes it necessary to provide many optical amplifiers, itresults in a generally expensive optical transmission system.

[0006] To deal with such problems, dispersion-management has beenproposed, wherein sections where the chromatic dispersion at thewavelength 1.55 μm is positive and sections where it is negative areprovided alternately in the longitudinal direction of the optical fibertransmission-line. If this kind of optical fiber transmission-line isused, by making the average chromatic dispersion in the optical fibertransmission-line as a whole substantially zero, it is possible tosuppress transmission quality deterioration caused by chromaticdispersion. And because at most points in the optical fibertransmission-line the absolute value of the chromatic dispersion is notin the vicinity of zero, it is thought to be possible also to suppresstransmission quality deterioration caused by four-wave mixing (see forexample U.S. Pat. No. 5,894,537 or 5,887,105).

[0007] However, the present inventors have recognized that in thisrelated art technology, when high-speed signal transmission with a bitrate exceeding 10 Gb/s is carried out using an optical fibertransmission-line wherein the alternating disposition of the sectionswhere the chromatic dispersion is positive and the sections where it isnegative is regular, side bands form around the original wavelength ofthe optical signal. This formation of side bands appears to be caused bythe interaction in the same pattern between the optical signal spectrumand the chromatic dispersion as a result of the regularity of thealternating disposition. And because this formation of side bandsconstitutes a cause of transmission quality deterioration, it isimportant that it be suppressed.

SUMMARY OF THE INVENTION

[0008] It is therefore an object of the present invention to provide adispersion-managed optical fiber transmission-line with which theformation of side bands around the optical signal wavelength can besuppressed even when carrying out high-speed signal transmission.

[0009] To achieve this object, a first optical fiber transmission-lineaccording to the invention is an optical fiber transmission-line forminga single repeater span in which sections where the chromatic dispersionat a predetermined wavelength is positive and sections where it isnegative are provided alternately, wherein the ratio between the maximumvalue and the minimum value among the absolute values of the averagechromatic dispersions of the sections is not less than 1.3 and notgreater than 10.0.

[0010] A second optical fiber transmission-line according to theinvention is an optical fiber transmission-line forming a singlerepeater span in which sections where the chromatic dispersion at apredetermined wavelength is positive and sections where it is negativeare provided alternately, wherein the number of sections of which theabsolute value of the average chromatic dispersion differs by not lessthan 10% from that of an adjacent section is at least half of the totalnumber of sections.

[0011] A third optical fiber transmission-line according to theinvention is an optical fiber transmission-line forming a singlereperter span in which sections where the chromatic dispersion at apredetermined wavelength is positive and sections where it is negativeare provided alternately, wherein the number of sections of which theabsolute value of the average chromatic dispersion differs by not lessthan 0.5 ps/nm/km from that of an adjacent section is at least half ofthe total number of sections.

[0012] A fourth optical fiber transmission-line according to theinvention is an optical fiber transmission-line forming a singlerepeater span in which sections where the chromatic dispersion at apredetermined wavelength is positive and sections where it is negativeare provided alternately, wherein for any two sections the absolutevalue of the average chromatic dispersion of the section nearer theoptical signal input end of the repeater span is larger than theabsolute value of the average chromatic dispersion of the section nearerthe optical signal output end and the absolute value of the averagechromatic dispersion of the section at the output end is not less than 1ps/nm/km. For any two sections in this fourth optical fibertransmission-line, the length of the section nearer the optical signalinput end of any repeater span is preferably shorter than the length ofthe section nearer the optical signal output end.

[0013] With any of the first through fourth optical fibertransmission-lines described above, the formation of side bands aroundthe optical signal wavelength can be suppressed even when carrying outhigh-speed signal transmission, and even when carrying out high-speedsignal transmission with a bit rate exceeding 10 Gb/s it is possible torealize WDM transmission of good transmission quality.

[0014] Also, with the fourth optical fiber transmission-line describedabove, it is not necessary for the sign of the chromatic dispersion tobe frequently alternated in regions where the absolute value of thechromatic dispersion is large, and the manufacturing productivity of theoptical fiber transmission-line is thereby improved.

[0015] And, in each of the first through fourth optical fibertransmission-lines described above, when the length of each section isnot less than 0.1 km and not more than 10 km, the cumulative chromaticdispersion does not become large, so the deterioration of transmissionquality caused by the interaction of cumulative chromatic dispersion andnonlinear optical phenomena can be suppressed. When the absolute valueof the average chromatic dispersion of each section is at least 1ps/nm/km, transmission quality deterioration caused by nonlinear opticalphenomena can be suppressed. When the absolute value of the averagechromatic dispersion of the whole repeater span is 0.5 ps/nm/km or less,transmission quality deterioration caused by cumulative chromaticdispersion can be suppressed. When the polarization mode dispersion ofthe whole repeater span is not greater than 0.2 ps/km^(½), transmissionquality deterioration caused by polarization mode dispersion can besuppressed. When the transmission loss is not more than 0.3 dB/km, thedistance of a transmission line without repeaters can be made greater.And when the effective core area is at least 20 μm² over the wholerepeater span, transmission quality deterioration caused by nonlinearoptical phenomena can be suppressed.

[0016] In this invention, a “predetermined wavelength” means the centerwavelength of an optical signal wavelength band, for example, 1.55 μm.And, when not otherwise specified, values of polarization modedispersion, transmission loss and effective core area are values at thepredetermined wavelength.

[0017] The above and further objects and novel features of the inventionwill be more fully clarified from the following detailed descriptionwhen the same is read in connection with the accompanying drawings. Itis to be expressly understood, however, that the drawings are for thepurpose of illustration only and are not intended as a definition of thelimits of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more fully understand the drawings used in thedetailed description of the present invention, a brief description ofeach drawing is provided.

[0019]FIG. 1 is a schematic view illustrating an optical fibertransmission-line according to any of first, second and third preferredembodiments;

[0020]FIG. 2 is a schematic view illustrating an optical fibertransmission-line according to a fourth preferred embodiment;

[0021]FIG. 3 is a graph illustrating another version of an optical fibertransmission-line according to the invention;

[0022]FIG. 4 is a graph illustrating another version of an optical fibertransmission-line according to the invention; and

[0023]FIG. 5 is a graph illustrating another version of an optical fibertransmission-line according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] In the following, the preferred embodiments of the presentinvention will be explained in detail with reference to the accompanyingdrawings. To facilitate comprehension of the explanation, the samereference numerals denote the same parts, where possible, throughout thedrawings, and a repeated explanation will be omitted. The dimensions inthe drawings are partly exaggerated and do not always correspond toactual ratios of dimensions.

[0025] (First Preferred Embodiment)

[0026] A first preferred embodiment of an optical fibertransmission-line according to the invention will now be described. FIG.1 is a view illustrating an optical fiber transmission-line 1 accordingto this first preferred embodiment. This optical fiber transmission-line1 constitutes one repeater span installed between a transmitter (orrepeater) 2 and a receiver (or repeater) 3, and is made up of N(N≧2)sections 4 ₁ through 4 _(N) in sequence from the transmitter 2 to thereceiver 3. At a predetermined wavelength the chromatic dispersion ispositive in the sections 4 _(n) where the value of n is odd and isnegative in the sections 4 _(n) where the value of n is even.

[0027] The optical fiber transmission-line 1 may be made by connectingtogether section by section with optical fibers having predeterminedchromatic dispersions, or the whole repeater span may be made from asingle optical fiber. In the latter case, the optical fibertransmission-line 1 can be obtained, for example, by the diameter of acore region being modulated to vary the chromatic dispersion while thediameter of the cladding region is kept constant along the longitudinaldirection. In this case the optical fiber transmission-line 1 can bemanufactured by making a preform in which the diameter of a core partvaries in the longitudinal direction and the diameter of the claddingpart is fixed in the longitudinal direction and drawing from thispreform an optical fiber whose cladding diameter is fixed. Or, theoptical fiber transmission-line 1 can be obtained by the diameter of thecore region and the diameter of the cladding region being modulated toadjust the chromatic dispersion in the longitudinal direction. In thiscase the optical fiber transmission-line 1 can be manufactured by makinga preform in which the respective diameters of the core part and thecladding part are fixed in the longitudinal direction and varying thecladding diameter in the longitudinal direction while drawing an opticalfiber from this preform. In either case, the larger the diameter of thecore region is, the larger the chromatic dispersion can be made.

[0028] The average chromatic dispersion of a section 4 _(n) will bewritten D_(n). Also, the maximum value among the absolute values ofD_(n) will be written Dmax, and the minimum value among the absolutevalues of D_(n) will be written Dmin. It is a characteristic feature ofthis preferred embodiment that the ratio R=Dmax/Dmin is at least 1.3 andnot more than 10.

[0029] If R is at least 1.3, then even when carrying out high-speedsignal transmission it is possible to suppress the formation of sidebands around the optical signal wavelength. And, because if the absolutevalue of D_(n) is at least 2 ps/nm/km in each section the occurrence ofnonlinear optical phenomena is suppressed and if the absolute value ofD_(n) is not greater than 20 ps/nm/km in each section the interaction ofcumulative chromatic dispersion and nonlinear optical phenomena issuppressed, the upper limit value of R is 10.

[0030] Table 1 shows Dmax (ps/nm/km), Dmin (ps/nm/km), R andtransmission characteristics for each of five different optical fibertransmission-lines A through E. TABLE 1 Transmission D max D min RCharacteristics Waveguide A 10 2 5 ◯ Waveguide B 20 2 10 ◯ Waveguide C2.4 1.8 1.3 ◯ Waveguide D 2.0 1.8 1.1 X Waveguide E Each absolute valueis X identical.

[0031] As shown in this table, in the optical fiber transmission-line A,Dmax was 10 ps/nm/km, Dmin was 2 ps/nm/km, and R was 5, and there was noformation of side bands. In the optical fiber transmission-line B, Dmaxwas 20 ps/nm/km, Dmin was 2 ps/nm/km, and R was 10, and there was noformation of side bands. In the optical fiber transmission-line C, Dmaxwas 2.4 ps/nm/km, Dmin was 1.8 ps/nm/km, and R was 1.3, and there was noformation of side bands. However, in the optical fiber transmission-lineD, in which Dmax was 2.0 ps/nm/km, Dmin was 1.8 ps/nm/km, and R was 1.1,side bands formed and the transmission characteristics deteriorated. Andalso in the optical fiber transmission-line E, in which the absolutevalue of D_(n) was the same in all the sections and R was 1.0, sidebands formed and the transmission characteristics deteriorated.

[0032] Thus, in this preferred embodiment, as a result of R beingbetween 1.3 and 10.0, the formation of side bands around the opticalsignal wavelength can be suppressed and good transmission quality can beobtained.

[0033] In this preferred embodiment, the length L_(n) of each of thesections 4 _(n) is preferably at least 0.1 km and not more than 10 km,and in this case, because the cumulative chromatic dispersion does notbecome large, transmission quality deterioration caused by interactionbetween cumulative chromatic dispersion and nonlinear optical phenomenacan be suppressed. The absolute value of D_(n) is preferably at least 1ps/nm/km, and in this case transmission quality deterioration caused bynonlinear optical phenomena can be suppressed. The absolute value of theaverage chromatic dispersion of the whole repeater span is preferablynot greater than 0.5 ps/nm/km, and in this case transmission qualitydeterioration caused by cumulative chromatic dispersion can besuppressed. The polarization mode dispersion of the whole repeater spanis preferably not greater than 0.2 ps/km^(½), and in this casetransmission quality deterioration caused by polarization modedispersion can be suppressed. The transmission loss of the wholerepeater span is preferably not greater than 0.3 dB/km, and in this casetransmission distances without repeater can be made long. And theeffective core area is preferably 20 μm² or more over the whole repeaterspan; in this case, transmission quality deterioration caused bynonlinear optical phenomena can be suppressed.

[0034] (Second Preferred Embodiment)

[0035] Next, a second preferred embodiment of an optical fibertransmission-line according to the invention will be described. Theconstruction of this second preferred embodiment is the same as thatshown in FIG. 1.

[0036] It is a characteristic feature of this preferred embodiment thatthe number of sections of which the absolute value of the averagechromatic dispersion differs by not less than 10% from that of anadjacent section is at least half of the total number N. That is, amongthe N sections 4 ₁ through 4 _(N) in the repeater span, the number ofsections of which the absolute value of D_(n) differs by at least 10%from the absolute value of D_(n−1) or the absolute value of D_(n+1) isat least N/2.

[0037] Table 2 shows chromatic dispersions D_(n) (units: ps/nm/km) ineach of two different optical fiber transmission-lines F and G. TABLE 2n 1 2 3 4 5 6 7 8 9 10 chromatic +11  −10  +8 −8 +6   −6   +4 −4   +2  −3 dispersions D_(n) of Waveguide F chromatic +5 −5 +3 −3 +2.5 −2.5 +2−2.5 +2.5 −2 dispersions D_(n) of Waveguide G

[0038] As shown in this table, in both of the optical fibertransmission-lines F and G, the number of sections of which the absolutevalue of the average chromatic dispersion differs by at least 10% fromthat of an adjacent section is at least half of the overall number N(=10), and even when carrying out high-speed signal transmission it ispossible to suppress the formation of side bands around the opticalsignal wavelength and good transmission quality can be obtained.

[0039] In this preferred embodiment also, all of the lengths L_(n) arepreferably at least 0.1 km and not more than 10 km; the absolute valueof every D_(n) is preferably at least 1 ps/nm/km; the absolute value ofthe average chromatic dispersion of the whole repeater span ispreferably not greater than 0.5 ps/nm/km; the polarization modedispersion of the whole repeater span is preferably not greater than 0.2ps/km^(½); the transmission loss of the whole repeater span ispreferably not greater than 0.3 dB/km; and the effective core area ispreferably 20 μm² or more over the whole repeater span.

[0040] (Third Preferred Embodiment)

[0041] A third preferred embodiment of an optical fibertransmission-line according to the invention will now be described. Theconstruction of the optical fiber transmission-line of this thirdpreferred embodiment is also the same as that shown in FIG. 1.

[0042] It is a characteristic feature of this preferred embodiment thatthe number of sections of which the absolute value of the averagechromatic dispersion differs by at least 0.5 ps/nm/km from that of anadjacent section is at least half of the total number of sections. Thatis, among the N sections 4 ₁ through 4 _(N) in the repeater span, thenumber of sections of which the absolute value of D_(n) differs by atleast 0.5 ps/nm/km from the absolute value of D_(n−1) or the absolutevalue of D_(n+1) is at least N/2.

[0043] Table 3 shows chromatic dispersions D_(n) (units: ps/nm/km) in anoptical fiber transmission-line H. TABLE 3 n 1 2 3 4 5 6 7 8 9 10chromatic +8 −8 +7.5 −7.5 +8.3 −6.0 +5.0 −7.3 +3.0 −3.0 dispersionsD_(n) of Waveguide H

[0044] As shown in this table, in the optical fiber transmission-line H,the number of sections of which the absolute value of the averagechromatic dispersion differs by at least 0.5 ps/nm/km from that of anadjacent section is at least half of the overall number of sections N(=10), and even when carrying out high-speed signal transmission it ispossible to suppress the formation of side bands around the opticalsignal wavelength and good transmission quality can be obtained.

[0045] In this preferred embodiment also, all of the lengths L_(n) arepreferably at least 0.1 km and not more than 10 km; the absolute valueof every D_(n) is preferably at least 1 ps/nm/km; the absolute value ofthe average chromatic dispersion of the whole repeater span ispreferably not greater than 0.5 ps/nm/km; the polarization modedispersion of the whole repeater span is preferably not greater than 0.2ps/km^(½); the transmission loss of the whole repeater span ispreferably not greater than 0.3 dB/km; and the effective core area ispreferably 20 μm² or more over the whole repeater span.

[0046] (Fourth Preferred Embodiment)

[0047] Next, a fourth preferred embodiment of an optical fibertransmission-line according to the invention will be described. FIG. 2is a schematic view of an optical fiber transmission-line according tothis fourth preferred embodiment. This optical fiber transmission-line 1constitutes one repeater span installed between a transmitter (orrepeater) 2 and a receiver (or repeater) 3, and is made up of N(N≧2)sections 4 ₁ through 4 _(N) in sequence from the transmitter 2 to thereceiver 3. At a predetermined wavelength the chromatic dispersion ispositive in the sections 4 _(n) where the value of n is odd and isnegative in the sections 4 _(n) where the value of n is even. D_(n) andL_(n) are defined in the same way as in the first preferred embodiment.

[0048] It is a characteristic feature of this preferred embodiment thatfor any two sections the absolute value of the average chromaticdispersion of the section nearer the optical signal input end of therepeater span is larger than the absolute value of the average chromaticdispersion of the section nearer the optical signal output end of therepeater span. That is, of any two adjacent sections 4 _(n), 4 _(n+1),the absolute value of the average chromatic dispersion D_(n) of thesection 4 _(n) nearer the input end is larger than the absolute value ofthe average chromatic dispersion D_(n+1) of the section 4 _(n+1) nearerthe output end. It is also a characteristic feature of this preferredembodiment that the absolute value of the average chromatic dispersionD_(N) of the section 4 _(N) at the output end of the repeater span is atleast 1 ps/nm/km.

[0049] In an optical fiber transmission-line having the chromaticdispersion of its sections set in this way, in sections nearer thetransmitter 2, where the power of the optical signal is relatively largeas a result of the absolute value of the chromatic dispersion beingrelatively large, the occurrence of nonlinear optical phenomena issuppressed. In sections nearer the receiver 3, where the power of theoptical signal is relatively small, because nonlinear optical phenomenado not readily occur, the absolute value of the chromatic dispersion canbe made small and the cumulative chromatic dispersion can thereby alsobe made small. And, by the absolute value of D_(N) being made at least 1ps/nm/km, the occurrence of nonlinear optical phenomena can besuppressed. In this preferred embodiment also, even when carrying outhigh-speed signal transmission, it is possible to suppress the formationof side bands around the optical signal wavelength and good transmissionquality can be obtained.

[0050] Also, preferably, for any two sections the length of the sectionnearer the optical signal input end of the repeater span is shorter thanthe length of the section nearer the optical signal output end of therepeater span, that is, in any two sections 4 _(n) and 4 _(n+1),L_(n)<L_(n+1). When this is done, in sections nearer the transmitter 2,as a result of the section length being relatively small, the cumulativechromatic dispersion can be made small. And in sections nearer thereceiver 3, as a result of the section length being relatively long,manufacturing productivity can be improved.

[0051] In this preferred embodiment also, all of the lengths L_(n) arepreferably at least 0.1 km and not more than 10 km; the absolute valueof every D_(n) is preferably at least 1 ps/nm/km; the absolute value ofthe average chromatic dispersion of the whole repeater span ispreferably not greater than 0.5 ps/nm/km; the polarization modedispersion of the whole repeater span is preferably not greater than 0.2ps/km^(½); the transmission loss of the whole repeater span ispreferably not greater than 0.3 dB/km; and the effective core area ispreferably 20 μm² or more over the whole repeater span.

[0052] (Modification Examples)

[0053] This invention is not limited to the preferred embodimentsdescribed above, and these preferred embodiments can be changed invarious ways within the scope of the invention. For example, althoughthe chromatic dispersion in each section of the optical fibertransmission-line may be uniform, as shown in FIG. 3, alternatively itmay change within the section, as shown in FIG. 4. In this latter case,for example if the sections 4 _(n) (of length L_(n)) where the chromaticdispersion is negative, are divided into sub-sections 4 _(ni) (i=1,2,3)of chromatic dispersion D_(ni) and length L_(ni), the average chromaticdispersion D_(n) of each such section 4 _(n) can be obtained from theformula D_(n)=(D_(n1)L_(n1)+D_(n2)L_(n2)+D_(n3)L_(n3))/L_(n). Indeed,provided that the average chromatic dispersion conforms to theprovisions of the invention, inside the sections the chromaticdispersion may be changed freely as long as its sign does not change.

[0054] Also, preferably, of any two adjacent sections 4 _(2m−1) and 4_(2m) (m=1 to N/2), it is preferable that the relationshipD_(2m−1)L_(2m−1)+D_(2m)L_(2m)=0 holds. In this case, because thecumulative chromatic dispersion over any two adjacent sections 4 _(2m−1)and 4 _(2m) is zero, waveform deterioration is suppressed.

What is claimed is:
 1. An optical fiber transmission-line constituting asingle repeater span in which sections where the chromatic dispersion ata predetermined wavelength is positive and sections where it is negativeare provided alternately, wherein the ratio between the maximum valueand the minimum value among the absolute values of the average chromaticdispersions of the sections is not less than 1.3 and not greater than10.0.
 2. An optical fiber transmission-line constituting a singlerepeater span in which sections where the chromatic dispersion at apredetermined wavelength is positive and sections where it is negativeare provided alternately, wherein the number of sections of which theabsolute value of the average chromatic dispersion differs by not lessthan 10% from that of an adjacent section is at least half of the totalnumber of sections.
 3. An optical fiber transmission-line constituting asingle repeater span in which sections where the chromatic dispersion ata predetermined wavelength is positive and sections where it is negativeare provided alternately, wherein the number of sections of which theabsolute value of the average chromatic dispersion differs by not lessthan 0.5 ps/nm/km from that of an adjacent section is at least half ofthe total number of sections.
 4. An optical fiber transmission-lineconstituting a single repeater span in which sections where thechromatic dispersion at a predetermined wavelength is positive andsections where it is negative are provided alternately, wherein for anytwo sections the absolute value of the average chromatic dispersion ofthe section nearer the optical signal input end of the repeater span islarger than the absolute value of the average chromatic dispersion ofthe section nearer the optical signal output end and the absolute valueof the average chromatic dispersion of the section at the output end isnot less than 1 ps/nm/km.
 5. An optical fiber transmission-lineaccording to claim 4, wherein for any two sections the length of thesection nearer the optical signal input end of the repeater span isshorter than the length of the section nearer the optical signal outputend.
 6. An optical fiber transmission-line according to any one ofclaims 1 through 4, wherein the length of each section is not less than0.1 km and not greater than 10 km.
 7. An optical fiber transmission-lineaccording to any one of claims 1 through 4, wherein the absolute valueof the average chromatic dispersion of each section is at least 1ps/nm/km.
 8. An optical fiber transmission-line according to any one ofclaims 1 through 4, wherein the absolute value of the average chromaticdispersion of the whole repeater span at the predetermined wavelength isnot greater than 0.5 ps/nm/km.
 9. An optical fiber transmission-lineaccording to any one of claims 1 through 4, wherein the polarizationmode dispersion of the whole repeater span at the predeterminedwavelength is not greater than 0.2 ps/km^(½).
 10. An optical fibertransmission-line according to any one of claims 1 through 4, whereinthe transmission loss of the whole repeater span at the predeterminedwavelength is not greater than 0.3 dB/km.
 11. An optical fibertransmission-line according to any one of claims 1 through 4, whereinover the whole repeater span the effective core area at thepredetermined wavelength is at least 20 μm².